Filters having a spiral winding are known in the prior art. DE 2 213 165 discloses a filter of this type, in which the filter cartridge consists of a multi-layer cloth which is wound up spirally and has two opposite, spiral end surfaces. The layers forming the cloth are sealed in such a manner that the fluid entering the filter cartridge through a spiral end surface has to flow through at least one filter surface before being able to leave the filter cartridge again. Each filter layer is located between two thin spacer layers having a high pore volume. At least one fluid-impermeable layer is placed against the surface of one of the spacer layers. By means of the use of a thin spacer layer, a compact filter cartridge having a large active filter surface per unit of volume is obtained.
U.S. Pat. No. 5,304,312 describes a sealed filter unit with a first and second end cap on the end side, wherein the end caps have connectors for the connection of lines for an untreated liquid to be filtered and for a permeate filtered out of said untreated liquid. The filter unit contains a filter element which is arranged between the end caps and consists of a spirally wound, two-layer filter composite material which comprises a filter layer and a liquid-impermeable spacer layer. At the two opposite end sides of the spirally wound filter composite material, the edges of the filter layer are each connected in a liquid-tight manner to the spacer layer adjacent on the left and right. This construction ensures that the untreated liquid supplied via the first end cap has to permeate the filter layer first in order to be removed as permeate at the second end cap.
EP 1 256 372 A2 (whose United States equivalent is United States Patent Publication No. 2002/168491A1) teaches a filter module which comprises a capillary filter mat wound spirally around a perforated pipe. An untreated fluid to be filtered is supplied through the perforated pipe and brought into contact with the spirally wound capillary filter mat. By means of a pressure differential between the inside and outside of the capillary filters, a permeate is filtered out of the untreated fluid and removed from the interior of the capillary filters. Access to the interior of the capillary filters is provided by the two end sides of the spirally wound capillary filter mat being provided with end caps made of a castable, curable material and the end caps obtained being cut to size substantially perpendicularly to the longitudinal axis of the capillary filters.
The filters which are known in the prior art and have spirally wound, multi-layer flat filter elements or capillary filter mats have one or more of the disadvantages mentioned below:                in order to separate untreated liquid and permeate, a liquid-impermeable layer is required, which increases the volume of the filter without contributing to the active filtration surface;        the individual layers of the flat filter elements are connected to one another only at the edges, and therefore have low mechanical stability;        due to the low mechanical stability, during the production and in particular during the winding to form a spiral, the flat filter elements are susceptible to creasing and warpage, and therefore the corresponding production methods are limited to small filter layer lengths.        